Did you make this instructable?

One reason may be your saliva. Saliva contains enzymes that break down starches, and starches are what make many "cream" soups thick. If you taste the soup, then put the remainder in the fridge, the enzymes from your saliva get into the soup, break down some of the starches and thin the soup. Prfesser

Thank you for this answer. It makes the most sense out of all of the answers that were given to the above question. I had ordered and eaten some of Panera bread's thick and creamy broccoli cheese soup at the restaurant. I took the remainder home and discovered, when I reopened it and it was now at slightly less than room temperature and that it had become quite thin and I was perplexed. Google took me to the above question and I found satisfaction in reading your answer.

I'm no scientist, but when hot (or warm) soup is refrigerated some moisure condenses on teh inside of the container. Also, sometimes when you let soup stand for a while it will seperate, try stirring the soup up.

Though as I write this I wonder why the condensation would make the composition less viscous at all. The moisture content should remain the same hot or cold. hmmm.... dilly of a pickle